Processes for the production of olefins wherein a crude oil, various residual oils and other heavy oils are thermally cracked by means of a fluidized bed of a particulate heat carrier, have heretofore been known, for example, from Japanese Patent Publication No. 36,289/1970.
The known apparatus for carrying out such processes are generally provided with a cyclone dust collector in order to reduce the loss of the particulate heat carrier. Most of the particles of the heat carrier, which have been accompanied by a stream of the reaction products and escaped from the reactor may be collected by the cyclone and returned to the fluidized bed.
In the thermal cracking of heavy oils, carbonaceous materials are normally by-produced to a great extent. A portion of such carbonaceous materials will deposit onto the particles of the heat carrier in the reactor, while the remaining portion will leave the fluidized bed. In the thermal cracking of heavy oils, the amount of the carbonaceous materials leaving the fluidized bed is of course larger than that in the thermal cracking of lighter oils, and it is unavoidable that such carbonaceous materials are deposited on the walls of various parts of the apparatus, located along the path between the reactor and a device for quenching the thermally cracked product. Such deposition and accumulation of carbonaceous materials are especially remarkable when the process is carried out at high temperatures as is the case with the process of the invention. For the purpose of producing olefins a temperature as high as 700.degree. to 850.degree. C. must be kept in the reactor.
Once such deposition of carbonaceous materials (generally referred to as "coking") has occurred on inside walls of the cyclone, protrusions and depressions are formed on inner surfaces of the walls which disturb the flow of gases in the cyclone and prevent solid particles to be collected by the cyclone from smoothly moving along the walls of the cyclone. Thus, the dust collecting ability of the cyclone is lowered to a great extent. On the other hand, in the thermal cracking of heavy oils for producing olefins, high temperatures must be kept in the reactor and at the same time the process must be carried out with the residence time of the cracked products in the reactor being as short as possible. Otherwise the products are excessively cracked to those of low value.
To realize the desired short residence time, the reactor must be operated with the highest possible linear velocity of gas in the fluidized bed and with the smallest possible space above the fluidized bed. As a result, on one hand, the fluidized state of the particles in the bed is so vigorous and non-uniform, that short period pressure variations in the reactor become considerably large, and, on the other hand, the interface between the fluidized bed and the space above the bed approaches the exit of the reactor so that the buffering function of the space is reduced. Consequently, the behavior of particles and the disturbance of gas in the fluidized bed directly affect the cyclone in that variations in the pressure difference between the bottom of the cyclone and the lower end of a duct for returning collected particles to the reactor become large, an the amount of gas flowing up through the duct to the cyclone increases. These factors, in addition to the above-discussed coking, further lower the dust collecting ability of the cyclone.
As discussed above, in a process for producing olefins by thermally cracking heavy oils with a fluidized bed carrier. owing to the required high temperature and short residence time, the dust collecting ability of the cyclone is inevitably lowered with time. Accordingly, when compared to other processes using a similar fluidized heat carrier under different conditions, the amount of particles of the heat carrier escaping the reactor, which is uncollected by the cyclone and passes to the steps of processing the reaction products, is considerably large, resulting in frequent troubles in operation.
As a particulate heat carrier, a particulate coke, sand, and finely divided ceramics may be used. The particles of the heat carrier forming the fluidized bed may be classified into two classes, one comprising coarse particles and which may readily be collected by a cyclone, and the other comprising fine particles which are inherently difficult to collect by a cyclone. The particles of the heat carrier, which are passed to the steps of the processing of the reaction products, are mainly comprised of the above-mentioned fine particles and a portion of the above-mentioned coarse particles having relatively small diameters. Accordingly, the particles of the heat carrier remaining in the reactor are of a relatively large size. Further, the deposition of cracked coke onto the particles make them larger. As a result, the fluidized state in the bed becomes intolerably non-uniform, and pressure variations in the reactor become large. These changes with time make it difficult to smoothly operate the apparatus. Furthermore, when the reactor comprises heating and reaction columns through which the fluidized heat carrier is recirculated, a smooth recirculation of particles between these columns is also prevented if the particles in the fluidized bed become excessively large.
In the addition to the above-discussed problems, the following inconvenience is also involved with respect to the material balance of the particulate heat carrier in the fluidized bed. In a reactor for thermally cracking heavy oils with the fluidized particulate heat carrier, while a coke is produced in the fluidized bed by the thermal cracking of heavy oils, the particles of the heat carrier are pulverized by the impingement and friction of the particles with each other and with the walls of the reactor, and the resultant fine particles may be withdrawn from the reactor accompanied by a stream of the reaction product. Particularly, when a particulate coke is used as a heat carrier, loss of heat carrier occurs partly due to the gasification of the coke by the reaction with steam in the reactor and partly due to a lowering in the dust collecting ability of the cyclone located at the exit of the reactor. As already discussed, the dust collecting ability of the cyclone is progressively lowered with time. As the loss of coarse particles of the coke due to a lowering in the dust collecting ability of the cyclone increases, the total loss of coke eventually exceeds the amount of coke produced even in the case where a heavy oil, which is likely to form a significant amount of coke, is used as a feed oil. Thus, it becomes necessary to supply an additional amount of particulate coke in order to maintain the required volume of the fluidized bed.
In order to cope with these circumstances, a quantity of the particulate heat carrier must frequently be withdrawn from the fluidized bed, pulverized, sieved and then returned to the bed to prevent the size of the particles in the bed from becoming excessively large; a fresh particulate heat carrier in an amount to compensate for the loss thereof must be added to the fluidized bed and; in order to prevent fine and coarse particles of the heat carrier which have passed to the steps of processing of the reaction product from depositing in parts along the path of the cracked oil, thus causing troubles in operation, these particles of the heat carrier must be separated from the cracked oil and further processed. However, it is not only extremely troublesome but also economically quite disadvantageous to frequently carry out the withdrawal, pulverization and returning of the particles and the supplying of fresh particles.
As already stated, the known apparatus are generally provided with a cyclone at the exit of the reactor in order to recover the solid particles accompanied by a stream of the reaction product. The primary object of providing the cyclone resides in the prevention of loss of particles as well as in the prevention of the contamination of reaction product with particles of the heat carrier suspended therein. Accordingly, in the prior art, fine particles of the heat carrier, which have been uncollected by the cyclone, are separated from the reaction product as follows. The reaction product containing such uncollected fine particles of the heat carrier is quenched to provide a cracked oil containing the particles of the heat carrier suspended therein; the oil is fractionated in a vacuum distillation column and; the resultant relatively high boiling oil containing the particles of the heat carrier is processed by means of a centrifuge or filter to remove the solid particles. However, some of the particles accompanied by the reaction product are so fine that it is difficult to completely remove them from the oil. Additionally the treatment of separated particles in troublesome since they are wetted with cracked oil.
Accordingly, it is an object of the invention to provide a solution to the above-mentioned various problems normally involued in a process for the thermal cracking of heavy oils with a fluidized particulate heat carrier.
Another object of the invention is to provide a process for the thermal cracking of heavy oil by which an improvement of the material balance of the particulate heat carrier and a desirably small change with time of the particle size distribution of the particulate heat carrier can be achieved and which can be continuously carried out for a prolonged period of time under substantially constant conditions.
A further object of the invention is to provide a process for the thermal cracking of heavy oils in which coking onto inner walls of the cyclone and its duct for returning collected particles to the reactor is eliminated or reduced and which may be carried out while maintaining a high level of the dust collecting ability of the cyclone.